The present invention relates to a shaft having magnetostrictive properties. The present invention further relates to a shaft with a magnetostrictive sensor for measuring a torque applied to a shaft, more particularly without contact and using the reverse-magnetostrictive effect of magnetic alloys. The present invention also relates to a method for making the same.
Ferromagnetic substances are characterized by showing slight changes in dimensions when they are magnetized and, conversely, by changing permeability when an outside force is applied to produce elastic deformation. The former is referred to as the magnetostrictive effect, and the latter as the reverse-magnetostrictive effect.
The saturation magnetostrictive constant lambda is used as an index for the degree of this effect. A sensor that magnetically detects torque applied to a rotating shaft using the above reverse-magnetostrictive effect is called a magnetostrictive torque sensor.
In shafts used in motors, machine tools and the like, the torque applied to the shaft is generally measured for the purpose of output control or of controlling changes in drive power. Magnetostrictive torque sensors are used for measuring this torque.
Known prior art magnetostrictive torque sensors include a magnetostrictive torque detector in which the shaft itself is composed of steel having a magnetic effect. An example is disclosed in Japanese laid-open publication no. 63-81993.
The prior art also includes shafts having a thin band of magnetic metal joined to the shaft surface with a synthetic resin adhesive or the like, forming a layer of magnetostrictive material as a magnetostrictive torque detector. An example of this type is disclosed in Japanese laid-open patent no. 63-163243. To effectively measure applied torque, the stress from the torque acting upon the shaft must be transmitted to the magnetostrictive material layer in order to detect torque from the shaft. Changes in permeability due to the reverse magnetostrictive effect of the magnetostrictive material layer may then be detected without contact from the outside.
However, the aforementioned prior art magnetostrictive torque sensors have the following problems. The former shaft having a magnetostrictive torque detector using the magnetostrictive effect of the shaft itself has a lower magnetostrictive effect compared to the latter shaft in which the magnetostrictive material layer is arranged separately, resulting in a lower sensitivity for torque detection. Thus, torque sensors using the former shaft involve complicated and expensive processing circuitry. In the latter shaft having a separately arranged magnetostrictive material layer, although corrosion resistance of the magnetostrictive material layer can be maintained, adequate adhesive strength between the magnetostrictive material layer and the shaft can not be obtained because the magnetostrictive material layer is bonded with a glue. Deterioration of the bond between the shaft and the magnetostrictive material layer is possible, particularly where the magnitude of the torque applied to the shaft is increased, or under environmental conditions of high temperature or high humidity. This results in decreased reliability.
Precision correlation between the applied torque and the detected change in permeability may be adversely affected in the latter prior art shaft. Since the ratio between the torque applied to the shaft and the binding strength of the synthetic resin adhesive decreases as the torque increases, adequate transmission of stress on the shaft to the magnetostrictive material layer is prevented. Further inaccuracies are believed to be caused by deterioration of the synthetic resin adhesive itself due to change over time and environmental operating temperature.
Additionally, Japanese laid-open patent publication no. 4-155232 discloses a method for producing a shaft with a magnetostrictive torque detector having a magnetostrictive material layer formed on the shaft surface without using adhesive. A nickel membrane is plated on the shaft surface, as a middle surface, upon which is plated a permalloy layer which is then heat treated. The use of a permalloy layer as the magnetostrictive material layer, as disclosed in this publication, allows relatively easy production with plating. However, the magnetostrictive constant of the permalloy layer is about 2-3.times.10.sup.-6. This is inferior in reverse magnetostrictive qualities compared to metallic de-crystallized magnetostrictive material layers, which have a magnetostrictive constant of 30-40.times.10.sup.-6. The plating method disclosed in this publication cannot be practicably implemented to bond the shaft and a ferrous de-crystallized magnetostrictive material layer having superior reverse-magnetostrictive qualifies, making the use of adhesive (as discussed above) a necessary step.